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If you’ve ever worked in a large organization, you’re probably already familiar with the Dunning-Kruger effect even if you don’t know the psychological term for the phenomenon. The immortal words of philosopher Bertrand Russell sum it up nicely: “The trouble with the world is that the stupid are cocksure and the intelligent are full of doubt.”

Or, to put it in the modern parlance, idiots don’t know they’re idiots, and the most skilled are generally also the most self-critical. You’ve probably observed this effect in many domains (driving and management maybe), but science recently added another entry to the roster of areas where people are often dangerously deluded.

Know that guy who claims to be a multitasking ninja? Yeah, in reality he’s probably abysmal at it.

“What is alarming is that people who talk on cells phones while driving tend to be the people least able to multitask well,” psychology Professor David Sanbonmatsu, a senior author of the study, commented. “We showed that people who multitask the most are those who appear to be the least capable of multitasking effectively.”

After giving 310 undergraduates a battery of tests to measure both their perceived multitasking skill and their actual ability, as well as collecting data on how much the test subjects multitasked out in the real world by driving while talking on a cell phone, the researchers concluded that those most drawn to multitasking usually lack the mental discipline and focus to do it relatively well.

“The people who multitask the most tend to be impulsive, sensation-seeking, overconfident of their multitasking abilities, and they tend to be less capable of multitasking,” co-author David Strayer said, elaborating that “people multitask because they have difficulty focusing on one task at a time. They get drawn into secondary tasks. … They get bored and want that stimulation.”

So what’s the takeaway for business owners? Don’t be taken in by appearances. A lot of multitasking on the part of an employee or job seeker shouldn’t be taken as evidence that they’re actually good at it or particularly skilled at regulating their attention.

In fact, if you see a job candidate pull into the car park while chatting away on their cell phone, probability says that guy is a rotten at multitasking.